Multivibrator frequency stabilized against variations in supply voltage and load



Feb. 21, 1967 E MOKEE 3,305,795

F. MULTIVIBRATOR FREQUENCY STABILIZED AGAINST VARIATIONS IN SUPPLY VOLTAGE AND LOAD Filed March 1, 1965 2 Sheets-Sheet l Q I l *8 '5 f a g &

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MULTIVIBRATOR FREQUENCY STABILIZED AGAINST VARIATIONS 2 Sheets-Sheet 2 IN SUPPLY VOLTAGE AND LOAD Filed March 1, 1965 Foo/7 2 f. M: //ee INVENTOR.

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A TTOIP/VEVJ United States Patent 3,305,795 MULTWIBRATQR FREQUENCY STABILIZED A-GAHNST VARIATIONS IN SUPPLY VOLT- AGE AND LOAD Fount E. McKee, Houston, Tex., assignor to Tideland Signal Corporation, a corporation of Texas Filed Mar. 1, 1965, Ser. No. 435,983 5 Claims. (Cl. 331113) The present invention relates to a multivibrator, and more particularly, relates to a free running multivibrator having a timing circuit which provides a constant frequency.

Generally, it is old to use a free running multivibrator having an RC type timing constant. However, the frequency of the multivibrator will generally not be of a constant frequency because of changes in the voltage supply and/or changes in the loading of the multivibrator.

It is a general object of the present invention to provide a free running multivibrator in which the output frequency is generally constant and independent of voltage changes.

A still further object of the present invention is the provision of a free running multivibrator wherein the timing capacitors are charged to a fixed voltage and attempt to charge back in the opposite direction to the same voltage to provide a constant timing circuit independent of voltage supply and loads.

Yet a further object of the present invention is the provision of a free running multivibrator circuit having first and second control switching elements which are connected together and alternately actuated by each other, each switching element having a control element, and providing a timing circuit for each of the control switching elements which includes a diode connected to the control element for allowing the passage of current toward the control element but not away from the control element, a timing capacitor one side of which is connected to the diode and the other side of which is connected to suitable means for charging the second side of the capacitor to a fixed voltage and a resistor connected to the other side of the capacitor for discharging the capacitor back to the same fixed voltage.

Other and further objects, features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure, and taken in conjunction with the accompanying drawings where like character references designate like parts throughout the several views, and where:

FIGURE 1 is an electrical schematic diagram of one embodiment of the present invention, and

FIGURE 2 is an electrical schematic diagram of a modified form of the present invention.

Referring now to the drawings, and particularly to FIGURE 1, the numerals and 12 generally designate first and second control switching elements, for example NPN transistors, which are conventionally connected together to form a free running multivibrator circuit. That is, the emitter 14 of transistor 10 is connected to the emitter 16 of transistor 12, the collector 18 of transistor 10 is connected to the base 20 of transistor 12 through a resistor 22, and the collector 24 of transistor 12 is connected to the base 26 of transistor 11) through a resistor 28. The emitters 14 and 16 are connected to one lead 30 of a voltage source and the collectors 18 and 24 are connected through resistors 32 and 34, respectively, to the voltage lead 36. Thus, as is conventional, the control switching elements or transistors 10 and 12 will alternately be turned on and off and will alternately actuate each other to provide an oscillating or free running multivibrator circuit when they are in turn connected to and actuated by a suitable timing circuit such as more fully described hereinafter.

The present invention is directed to providing an RC timing circuit for controlling the frequency of operation of the multivibrator so as to provide a relatively con stant frequency by providing a timing circuit which will operate independently of changes in the supply voltage across the terminals 36 and 30 or changes in the load circuits of the transistors 10 and 12.

Thus, symmetrical timing circuits are connected to each of the bases 26 and 20 of the transistors 10 and 12 to control the frequency of operation. Diodes 38 and 40 are provided connected to the bases 26 and 20, respectively, to allow the passage of timing signals to the bases of the transistors 10 and 12, but prevent any control signals from the resistors 28 and 22, respectively, to flow towards and interfere with the timing circuits. Timing condensers 42 and 44 are provided connected to the bases 26 and 20 of the transistors 10 and 12 through the diodes 38 and 40, respectively. Suitable means are provided connected to the sides of the capacitors 42 and 44 remote from the bases 26 and 20, respectively, for charging the second side of the condensers positively. Thus, suitable switching means, such as transistors 46 and 48, may be provided with their emitters 50 and 52, respectively, connected to the second sides of the condensers 42 and 44. The collectors 54 and 56 of the transistors 46 and 48, respectively, are connected to the positive line voltage 36. Thus, when the transistors 46 and 48 are turned on they will charge the second side of the capacitors 42 and 44, respectively, to the value of the positive voltage at line 36. The transistors 46 and 48 are alternately actuated with the base 58 of transistor 46 controlled through resistor 62 by the collector 24 of transistor 12. Similarly, the base 60 of transistor 48 is controlled through resistor 64 by the collector 18 of transistor 10. Thus, a negative voltage is alternately provided on the first side of the condensers 42 and 44 and a negative voltage is alternately provided at the bases of the transistors 10 and 12 holding these transistors alternately in the off position. The condensers 42 and 44 attempt to charge in the opposite direction through resistors 66 and 68, and 70 and 72, respectively. Generally, the resistors 68 and 72 are much larger than the resistors 66 and 70 and therefore the charging time is primarily controlled by the resistors 68 and 72. The alternately charged condensers 42 and 44 will attempt to charge toward the value of the supply voltage at the terminal 36, but when the voltage across the condensers reaches the turn on point, which for the transistors 10 and 12 will be zero potential or just slightly above zero potential, they will alternately cause the transistors 10 and12 to be turned on.

The off time of transistor 10 is determined by resistors 66 and 68 and the condenser 42. The off time of transistor 12 is determined by resistors 72 and 70 and condenser 44. The equation for the timing circuits is generally as follows:

t: RC 1n E-turn on voltage) For the transistors 10 and 12 the turn on voltage is approximately zero therefore the equation for the timing of this circuit is as follows:

is to be noted that since the condensers 42 and 44 are always charged to a fixed voltage, in this case the voltage at line 36, and attempt to charge back to that same voltage, the frequency of the timing circuit is independent of the supply voltage. And of course the diodes 38 and 40 prevent any changes in the loading circuit of the transistors and 12 from affecting the discharge time of the condensers 42 and 44. The output circuit may be connected to either or both of terminals 41 and 43.

Of course, various modifications of the timing circuits may be utilized to insure that the timing condensers are alternately charged to a certain voltage and attempt to charge back to the same voltage for controlling the frequency of the multivibrator. FIGURE 2 illustrates a modification of the timing circuit according to the present invention, the letter a being applied to the parts corresponding to those in FIGURE 1 for convenience of reference. Thus, referring now to FIGURE 2 the free running multivibrators 10a and 12a are connected as before along with the diodes 38a and 40a and timing condensers 42a and 44a.

However, in this embodiment the second side of the timing condensers 42a and 44a are charged positively from the line 36a through resistors 80 and 82 respectively. However, switching means, such as transistors 84 and 86, are provided between the second side of the condensers 42a and 4411, respectively, which when actuated provides a circuit through the resistors 80 and 82 respectively, to allow charging of the condensers 42a and 44a, respectively, in the opposite direction. Since the purpose of switches 84 and 86 is to allow charging of the condensers in the opposite direction from that in FIGURE 1, the control elements or bases 88 and 92 of the transistors 84 and 86 are connected to the opposite multivibrator switching element from that in FIGURE 1. Thus, the base 88 of the transistor 84 is controlled through resistor 90 from the collector 18a of transistor 10a. Similarly, the base 92 of transistor 86 is controlled through resistor 94 from the collector 24a of transistor 12a. Again, the equation for the timing of this circuit is given by the following formula:

where for the timing condenser 42a the resistance R is 68a and for the timing condenser 44a the resistance R is the resistor 72a.

In operation, and referring now to FIGURE 1, assuming that the circuit has been initially started by a suitable starting circuit which is conventional and forms no part of the present invention, assume that transistors 12 and 48 are on and transistors 10 and 46 are off. With transistor 48 on, the second side of condenser 44 will be charged through the transistor 48 to a positive value of the voltage in line 36. The current through resistors 22 and 72 provide a voltage at base to hold the transistor 12 in the on position. However, since transistors 10 and 46 are now off, they hadpreviously been on in the previous cycle and had charged the second side of condenser 42 to a positive voltage equal to the supply volt- :age at terminal 36. Thus, when transistors 10 and thus 46 are turned off, condenser 42 would have a charge which provides a negative voltage to the base of transistor 10 thus holding the transistor 10 in the off position. Condenser 42 now attempts to charge in the opposite direction through resistors 66 and 68 towards the value of :the supply voltage. In addition, the voltage on condenser 42 is applied through diode 38 to the base 26 of transistor 10 thereby controlling the cutoff of transistor 10. When the voltage across condenser 42 reaches the turn on level for transistor 10, which is generally just slightly above zero potential, transistor 10 will again be turned on. When transistor 10 turns on, transistors 12 and 43 will be turned ofi since they are controlled by the voltage at collector 18 of transistor 10 and transistor 46 will be turned on as the voltage on collector 24 of transistor 12 is no longer negative. Thus, the alternate half of the cycle now begins as condenser 44 which has previously been charged positive on the second side now tries to charge through resistors and 72, and will continue to charge until it reaches the turn on point for transistor 12 at which time the voltage on condenser 44, which is applied through diode 40 to the base 20 of transistor 12, will again turn transistor 12 on to repeat the cycle.

Referring now to the operation of the free running multivibrator in FIGURE 2, and assuming that transistors 10a and 86 have just been turned off and that transistors 12:: and 84 are on. With transistor 86 turned off condenser 44a will be charged to a positive valve through the resistor 82. The current through resistors 22a and 72a provides a voltage at the base of transistor 12a to hold transistor 12a in the on position. Since transistor 10a has now been turned off and it has previously been on and transistor 84 has been off, it had allowed condenser 42a to be charged to the value of the supply voltage through resistor 80. However, when transistor 10a was turned off transistor 84 was turned on, leaving condenser 42a to be charged to provide a negative voltage through diode 38a to the base of transistor 10a thus holding the transistor 10a in the off position. Condenser 42a now tries to charge in the opposite direction through resistor 68a and will charge toward the value of the supply voltage and when that voltage reaches the turn on potential of the transistor 10a, which will be just slightly above zero potential, transistor 10a will again be turned on thereby supplying a negative voltage from its collector to the bases of transistors 12a and 84 turning off transistors 12a and 84 and with transistor 86 being turned on as the negative voltage at the collector of transistor 12a is cut off from the base of transistor 86. Condenser 44a which was charged during the time that transistor 86 was off now tries to charge through resistor 72a and will continue to charge until its charge which is applied through diode 40a to the base of transistor 12a, reaches a value slightly above zero potential at which time it will turn transistor 12a on thereby repeating the cycle.

The off time of transistor 10a is determined by resistor 68a and condenser 42a. The off time of transistor 12a is determined by resistor 72a and condenser 44a. Since the condensers 42a and 44a are always charged to the value of the supply voltage, and discharged to approximately zero volts, the time required for the discharge of condensers 42a and 44a to zero volts is independent of the value of the supply voltage or of the load voltage of the multivibrator. This makes the type of timing circuit shown desirable where a reasonably constant frequency or constant timing circuit is desired and the supply voltage or load voltage fluctuates.

The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned as well as others inherent therein. While presently preferred embodiments of the invention are given for the purpose of disclosure, numerous changes in the details of construction and arrangement of parts may be made which will readily suggest themselves to those skilled in the art and which are encompassed within the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. In a free running multivibrator circuit having first and second control switching elements which are connected together and alternately actuated by each other, a first control element connected to the first switching element, and a second control element connected to the second switching element, the improvement comprising,

a first diode connected to the first control element passing current towards, but not away from said first control element,

a second diode connected to the second control element passing current toward, but not away from said second control element,

a first timing capacitor one side of which is connected to the first diode remote from the first control element,

a second timing capacitor one side of which is con-- nected to the second diode remote from said second control element,

a third control switching means connected to said first capacitor on the side remote from said first diode and connected to a voltage source for controlling the charging of said first capacitor, said third switching element connected to and controlled by one of the first and second control switching elements,

a fourth control switching means connected to said second capacitor on the side remote from the second diode and connected to a voltage source for controlling the charging of said second capacitor, said fourth switching means connected to and controlled by the other of the first and second control switching element,

a first resistance means connected to said one side of the first capacitor and said voltage source, and

a second resistance means connected to said one side of the second capacitor and said voltage source.

2. In a free running multivibrator circuit having first and second controlled switching elements which are connected together and alternately actuated by each other, a first control element connected to said first switching element and a second control element connected to the second switching element, the improvement comprisig,

a first diode connected to the first control element,

a second diode connected to the second control element,

a first capacitor connected to the first diode, remote from the first control element,

a second capacitor connected to the second diode, re-

mote from the second control element,

first and second resistors connected to opposite sides of said first capacitor and between a voltage source,

third and fourth resistors connected to opposite sides of said second capacitor and between a voltage source,

a third controlled switching element having a third control element and connected to said first capacitor on the side remote from the first diode and to the voltage source for charging said capacitor, the third control element connected to the first control element,

a fourth controlled switching element having a fourth control element and connected to said second capacitor on the side remote from the second diode and to the voltage source for charging said capacitor, the fourth control element connected to the second control element.

3. In a free running multivibrator circuit having first and second switching transistors which are connected together and alternately actuated by each other, each switching transistor having a control base, the improvement in a timing circuit for connection to each one of the control bases comprising,

a diode for connection to the base,

a capacitor one side of which is conneted to the diode remote from the base,

first and seond resistors connected to opposite sides of said capacitor and to a voltage source,

a capacitor charging transistor connected to the side of the capacitor remote from the diode and to said voltage source, said capacitor charging transistor having a control base connected to and controlled by the opposite switching transistor.

4. In a free running multivibrator circuit having first and second transistors which are connected together and alternately actuated by each other, each having a control base, the control base of one being connected through a bias resistor to the collector of the other, the improvement in a timing circuit for each of the transistors for connection to and actuation of the base voltage of each transistor comprising,

a diode connected to the base for passing current to the base but limiting current flow through said diode,

a timing capaictor having a first side connected to the diode remote from the base,

first and second timing resistors the first ends of which are connected to opposite sides of said capacitor and the second ends of which are connected across a voltage source,

a charging transistor having its emitter connected to the condenser remote from the diode, its collector to said voltage source, and its base to the collector of the opposite transistor.

5. In a free running multivibrator circuit having first and second transistors which are connected together and alternately actuated by each other, each transistor having a control base, the improvement in a timing circuit for each of the transistors and connected to each of the control base comprising,

a diode for connection to the control base allowing the passage of current toward but not away from the control base,

a timing capacitor one side of which is connected to the diode remote from said base,

a resistor connected to said one side of the capacitor and to a voltage source,

a source of voltage connected to the second side of said capacitor,

a switching transistor connected to the second side of said capacitor for discharging said capacitor when actuated, and

said switching transistor controlled by the transistor to which the capacitor is connected.

References Cited by the Examiner I UNITED STATES PATENTS 3/1956 Trousdale 331-113 6/1962 Regis 331113 

1. IN A FREE RUNNING MULTIVIBRATOR CIRCUIT HAVING FIRST AND SECOND CONTROL SWITCHING ELEMENTS WHICH ARE CONNECTED TOGETHER AND ALTERNATELY ACTUATED BY EACH OTHER, A FIRST CONTROL ELEMENT CONNECTED TO THE FIRST SWITCHING ELEMENT, AND A SECOND CONTROL ELEMENT CONNECTED TO THE SECOND SWITCHING ELEMENT, THE IMPROVEMENT COMPRISING, A FIRST DIODE CONNECTED TO THE FIRST CONTROL ELEMENT PASSING CURRENT TOWARDS, BUT NOT AWAY FROM SAID FIRST CONTROL ELEMENT, A SECOND DIODE CONNECTED TO THE SECOND CONTROL ELEMENT PASSING CURRENT TOWARD, BUT NOT AWAY FROM SAID SECOND CONTROL ELEMENT, A FIRST TIMING CAPACITOR ONE SIDE OF WHICH IS CONNECTED TO THE FIRST DIODE REMOTE FROM THE FIRST CONTROL ELEMENT, A SECOND TIMING CAPACITOR ONE SIDE OF WHICH IS CONNECTED TO THE SECOND DIODE REMOTE FROM SAID SECOND CONTROL ELEMENT, 